The present invention relates to gas turbine engines. More particularly, the present invention relates to vibration damping for a bearing support assembly for a gas turbine engine.
Squeeze film damping can be used in a bearing support assembly that supports a rotatable shaft of a gas turbine engine. The bearing is contained within a stationary housing. A bearing centering spring, also known as a squirrel cage, supports the bearing. The squirrel cage is connected to the stationary housing such that the squirrel cage does not rotate relative to the stationary housing, yet the squirrel cage is flexible enough to bend in response to loads experienced from the bearing. A film of liquid, such as oil, is supplied to an annulus disposed between the stationary housing and the squirrel cage. The film of oil reduces or damps engine vibration and the transmission of vibrations from the bearing to the engine structure.
When the compressor or turbine shaft rotates, imbalance or rotor instabilities can cause excessive whirling of the shaft and vibration of the engine, engine mounts and airframe. As a speed of the shaft increases, the rotating shaft passes through a variety of critical speeds, which occur when the speed of the rotating shaft traverses a resonant frequency of one of the various structures of the engine. Due to the multitude of structures that make up any given engine, the rotating shaft passes through several critical speeds as the rotating shaft accelerates. As the rotating shaft rotates, the rotating shaft may be displaced from a centerline of the engine. The rotating shaft then whirls about a centerline of the engine. As the bearing moves with the rotation of the shaft, the squirrel cage vibrates within the stationary housing and squeezes the oil in the annulus. Squeezing the oil in the annulus creates a pressure wave that may comprise a damping force, when the oil shears at an angle orthogonal to the displacement of the shaft from the engine centerline, or a stiffness force, when the oil shears along the direction of displacement or opposite the direction of displacement. When the oil acts as a stiffener, the oil actually increases the transmission of vibrations to the engine and airframe structure. In such a case, the vibrations experienced by the bearing assembly must be damped in another manner.